Gear differentials generally include compound planetary gear sets interconnecting a pair of drive axles to permit the latter to rotate in opposite directions with respect to a differential housing. The drive axles rotate about a common axis; and a pair of respective sun gears (often called "side" gears) are fixed for rotation with the inner ends of the two drive axles, such gears acting as the sun gear members of the compound planetary gear sets. The side gears are interconnected by the planet gear members of the sets. The planet gears (sometimes referred to as "element" or "combination" gears) are most commonly arranged as sets of meshing pairs, being spaced circumferentially and equidistant about the common axis of the sun gears (e.g., four pairs arranged at 90.degree. intervals or three pairs at 120.degree. intervals); and the planet gears may be mounted for rotation about axes that are parallel to, or variously offset and inclined with respect to, a common axis of the sun gears and drive shafts. My invention relates to "parallel-axis" differentials in which the planet gears are mounted on axes parallel to the common axis of the side gears.
The entire planetary gearing arrangement within the differential housing supports opposite relative rotation between the drive axle ends (i.e., differentiation), which is necessary to permit the axle ends to be driven at different speeds. Torque transmitted to the drive axles through the inclined tooth surfaces of the sun/side gears generates thrust forces against gear-mounting bearing surfaces within the differential. (Such bearing surfaces may comprise journals formed in the housing, or may be the ends of bores into which the gears are received, or may be special washers positioned between the end faces or shaft ends of the gears and the housing.) The thrust forces, together with other loads conveyed by the gear meshes in the planetary gearing, produce a frictional resistance to relative rotation between the drive axles, this frictional resistance being proportional to the torque applied to the differential housing. The proportional frictional resistance supports different amounts of torque between the two drive axles to prevent their relative rotation until the characteristic "bias" ratio of the planetary gearing arrangement is reached. Once the frictional resistance is overcome and differentiation begins, the torque difference between the axles is proportioned in accordance with the bias ratio. Differentials that divide torque in a substantially constant ratio between relatively rotating drive axles are referred to as "torque-proportioning" differentials.
The ability to support different amounts of torque between the drive axles is of great benefit to improving traction capabilities of vehicles. Ordinarily, when one wheel of a vehicle with a conventional differential loses traction, the amount of torque that can be delivered to the other drive wheel is similarly reduced. However, when one wheel loses traction so that there is differentiation between the two axles, torque-proportioning differentials deliver an increased amount of torque to the drive wheel having better traction, such increased torque being determined by the characteristic bias ratio of the differential.
In typical parallel-axis torque-proportioning differentials (e.g., U.S. Pat. Nos. 2,269,734 to L. S. Powell and 3,706,239 to A. F. Myers), each planet gear is in mesh with a paired planet gear, and each planet gear in the pair meshes, respectively, with one of the side gears; and one axial end of each individual planet gear is in mesh with its respective side gear, while its other axial end is in mesh with its paired planet gear. That is, in most parallel-axis torque-proportioning differentials, the planetary gear pairs mesh with each other at only one of their axial ends, and their respective loads are often carried primarily by only one end of their axial mounting supports.
However, one parallel-axis differential of more recent design (commonly owned U.S. Pat. No. 5,122,101 to G. B. Tseng) provides such differentials with an increase in frictional surfaces and greater control over bias ratio. In this recent design, the paired planetary gears of each circumferentially-spaced set mesh with each other at two separated areas of engagement. That is, each combination gear of the pair is in mesh with a respective one of the side gears, and each shares two separate and distinct meshing areas with its paired combination gear. For each combination gear, the two meshing portions shared with its paired gear "straddle" the portion of the gear which is in mesh with its respective side gear. Preferably, the shared mesh portions are located at the two axial outer ends of the combination gears. This arrangement also improves the load balance on the planetary gear mounting supports.
As indicated above, when torque is transmitted to the drive axles through the inclined tooth surfaces of the sun/side gears, thrust forces are generated within the differential. Some of these axially-directed thrust forces cause the confronting end faces of the side gears and/or of their respective drive axles to be directed against each other. In prior art parallel-axis differentials, blocks are often positioned between these confronting end faces, being used (a) with spring bias to increase the frictional resistance to relative rotation between the axles (e.g., U.S. Pat. No. 3,375,736 to O. E. Saari), and/or (b) to maintain the position and separation of the confronting faces (e.g., U.S. Pat. No. 4,365,524 to W. L. Dissett et al).
However, such separating block arrangements are relatively complex and add to the cost of manufacturing and maintaining the differential; and therefore, most commercially-acceptable designs of torque-proportioning differentials do not incorporate such relatively expensive separating arrangements. Nonetheless, the confronting end faces of the side gears are still subject to undesirable wear, and it would be preferable to minimize such wear if it could be achieved in a simple and economical manner.
These confronting end faces are also involved in the following basic design problem: To achieve certain desired torque-bias characteristics in many popular differential designs, torque-proportioning differentials of both parallel-axis and inclined-axis types often provide the side gears with helix angles inclined in the same direction with respect to their common axis of rotation, causing both side gears to be thrust toward the same side of the housing whenever power is applied to the housing to effect forward motion on the vehicle. With this gearing arrangement, when differentiation causes torque to be transferred between the axles in a first direction of relative rotation (e.g., from the left axle to the right axle), the transfer is opposed by a first bias ratio; however, when torque is transferred between the axles in the opposite direction of relative rotation (e.g., from the right axle to the left axle), the transfer is opposed by a different bias ratio.
To help overcome this unequal bias problem in inclined-axis differentials, commonly owned U.S. Pat. No. 4,890,511 (H. Pedersen) proposes the insertion of a non-rotatable washer element between the confronting faces of the side gears, the washer element being provided (a) with radially outwardly directed projections which engage the main body portion of the housing to prevent rotation of the washer element, and (b) with different coefficients of friction on its opposite side surfaces to reduce and/or control such bias-ratio differences.
The just-described Pedersen washer element is easily adapted into differentials of the crossed-axis design. However, parallel-axis differentials in present use commercially have housing and gear arrangement designs that would require significant modifications in order to accommodate bias-adjusting washers according to the teachings of the Pedersen reference. Therefore, in spite of the fact that parallel-axis differentials continue to be affected by the unequal bias ratio problem as well as by undesirable wear at the confronting end faces of the side gears, I am unaware of any prior art disclosing an economical, simple-to-manufacture washer element which can be easily installed in present commercial designs of parallel-axis differentials to provide a thin, non-rotating barrier between the confronting end faces of the side gears.
My invention addresses these parallel-axis design problems.